1. Field of the Invention
This invention relates to improvements in electronic flash devices for cameras having an auxiliary power supply switch (or "auxiliary switch") arranged to close when the device is attached to the camera.
2. Description of the Prior Art
Electronic flash devices of the kind described generally have an electrical circuit as shown in FIG. 1. The voltage of a battery 1 is converted to an alternating current of increased voltage by a booster I. Booster I includes a battery capacitor 2, an oscillating transistor 3, a bias capacitor 4, an oscillating transformer 5 having a saturated iron core, and a resistor 6. The output of booster I passes through a high voltage rectifier diode 7 to a main or storage capacitor 8. Use of a manually operated main switch 9 is generally sufficient to initiate operation of the booster circuit I. However, if the power supply is controlled only by the main switch 9, a problem arises. When the electronic flash device is not used for a long time and is detached from the camera, while leaving the battery switch 9 ON, the booster circuit I continues to operate, causing wasteful consumption of the electrical energy of the battery. One proposed, and utilized, way of avoiding this has been to provide this kind of electronic flash device with an auxiliary battery switch 10 arranged to open or close when the device is detached from or in use with the camera.
FIG. 2 illustrates the construction of such an auxiliary switch 10. An embossed portion is formed on the inner surface of a casing of the electronic flash device. A print substrate 102 fixed by the fastener screws 101 is mounted on the embossed portion. A switch contact in the form of a plunger rod 10s passes through a fitted hole provided through the wall of the print substrate 102 and is urged by a coil spring 103 to move away from a fixed contact 10a when the device is detached from the camera. The coil spring 103, plunger 10s and a copper foil pattern 10c of the print substrate 102 are always electrically connected with one another, constituting a movable contact of the switch 10. When the device is attached to the camera, end "a" of the plunger 10s is brought into electrical connection with a terminal on the camera housing (not shown). At the same time, the opposite end of plunger 10s contacts the resilient contact member 10a. The auxiliary switch 10 is thus turned on. When the main switch 9 is then turned on with movable contact 9s in contact with a contact 9a, the booster circuit I starts to oscillate. When the device is then detached from the camera, auxiliary switch 10 is turned off and operation of booster circuit I is automatically stopped, even when the main switch 9 is still in the ON state.
Current induced in the secondary winding S of the oscillation transformer 5 flows in a closed circuit from the high voltage rectifier diode 7 through the storage capacitor 8, closed main switch 9, closed auxiliary switch 10, battery 1, capacitor 2 connected in parallel to the battery 1 and the emitter-to-base path of oscillating transistor 3 to the opposite end of the secondary winding S. With the electronic device on the camera, this closed circuit changes to the open position when the main switch 9 changes from its ON to OFF state, or when chattering takes place at the time of contact between the members 9s and 9a of the main switch.
Current flowing through the closed circuit is divided into two parts, one of which serves as a charging current for the storage capacitor 8. Another part flows through the emitter-base path of oscillating transistor 3, becoming a positive feedback current to booster circuit I. Because the opening of this closed circuit results in an increased impedance between the lines A and B, a surge voltage is produced for a very short time across lines A and B. The higher the impedance of the circuit beginning with the base of oscillating transistor 3 with resistor 6 and terminating at the main switch contact 9c, inevitable for actuating booster circuit I, the more prominent the surge voltage becomes. Such surge voltage does not create any problem provided that the synchronizing switch for controlling the timing of firing of the electronic flash device is in the form of a mechanical switch 11, as shown in FIG. 1. This prevails in the prior art.
For cameras employing a CMOS type integrated circuit of low withstand voltage from which an actuating signal for firing the device is obtained, however, the production of a surge voltage across lines A and B becomes a serious problem, because the surge voltage is applied backwards through the interconnection terminal leading from line B to the CMOS type integrated circuit and from there to line A. It is therefore unavoidable that the integrated circuit and associated parts therewith are broken as the withstand voltage is exceeded.
To prevent such withstand voltage damage, a diode of high withstand voltage may be connected in such a direction to not apply the surge voltage directly to the integrated circuit. With the reduced driving voltage for the integrated circuit, however, the output signal of the diode is lowered by the voltage loss (about 0.6 volts) in the forward direction to a value depending on the ambient temperature and an alternative problem of securing the accuracy and reliability of control results. The use of such a diode cannot be said to be an advantageous solution.